The present invention relates to a controlled-growth mercury drop electrode for use in electrochemical experiments, and more particularly to a valve control for regulating the rate of growth and surface area of the mercury drops. The electrochemical experiments include measurement of, for example, surface tension, current at controlled potential, potential at controlled current or charge, and so on. Voltammetry, a group of techniques involving measurement of current as a function of potential with time as a parameter, is discussed herein as an example of a specific use of the present invention.
In voltammetry, the potential of the working electrode, here a droplet of mercury, is maintained at a known value with respect to a reference electrode placed in the same solution. An electroactive substance in the solution will transfer electrons to, or accept electrons from, the external circuit at the surface of the working electrode if the potential is in a characteristic range. The former process is described as anodic oxidation, the latter as cathodic reduction. The magnitude of the current is proportional to the concentration of the substance in solution; thus voltammetry is used for quantitative analysis. The characteristic potential depends on the identity of the substance; thus it is also used for qualitative analysis.
The volume of solution is typically 1-50 mL, but the electrochemical reaction typically takes place only near the working electrode, and thus the technique is nondestructive. In routine use, the concentration of electroactive material can be determined in the range 10.sup.-8 to 10.sup.-2 M Specialized techniques such as stripping voltammetry permit determinations down to 10.sup.- M in some cases.
Voltammetry is usually carried out with a three-electrode configuration. A potentiostat is employed to control the potential of the working electrode with respect to the reference electrode by forcing the necessary current through an auxiliary electrode. This current also passes through the working electrode and is measured using a current-to-voltage transducer. The control potential is varied according to a specific potential-time program which may consist of a ramp, sine wave, or pulse sequence, for example. The current is sampled, differenced, averaged, or subjected to some other manipulation appropriate to the potential-time program to produce a current output. A plot of current output versus some function of the control potential is called a voltammogram.
Voltammograms are generally S-shaped or peak-shaped. The S-shaped voltammograms are characterized by a limiting current, i.sub.1, and a half-wave potential, E.sub.1/2, (potential at which i=i.sub.1 /2). Peak-shaped voltammograms are characterized by peak current, i.sub.p, and peak position, E.sub.p. Either i.sub.1 or i.sub.p is a measure of the concentration of reacting material, and E.sub.1/2 or E.sub.p depends on the fundamental properties of the charge transfer process.
Current within the working electrode may arise from anodic or cathodic reaction or from the process of charging the electrode surface, which behaves like a potential-dependent capacitor. Thus, changes in potential or area require charging current which appears as an unwanted component of the total current when the current due to charge transfer is sought. When mercury is used as the electrode material, it is desirable to measure current at fixed area in order to eliminate the charging current which would arise due to change in area. The charging current due to change in potential decays exponentially with the time constant RC, R being the resistance and C the capacitance of the electrode. Smaller values of RC permit reliable current measurements at shorter times. This is desirable because the current due to charge transfer also decays with time; thus, measurements at shorter times give more signal per unit concentration.
Mercury is a highly desirable electrode material because the liquid surface is readily and reproducibly renewable, in sharp contrast with solid electrode materials. It also has an excellent potential range for carrying out reductions in aqueous solution.
Due to their advantages, mercury drop electrodes are widely used in the field of electrochemical analysis. Prior ar devices have a number of disadvantages, however, which are well-documented in the prior art. For example, to ensure optimum voltammetric results, it is critical to maintain electrical continuity between the mercury in the reservoir and the mercury in the capillary. The resistance of this contact between the mercury in the capillary with that in the reservoir usually is the limiting factor in extending applications of the static mercury drop electrode to measurements at short times. In an attempt to solve this problem, one prior art design includes coating the tip of the capillary with a layer of tin oxide to ensure continuity. However, the mechanical design of the valve of this prior art electrode is such that the relatively massive plunger repetitively pounds the tip of the capillary leading to a deterioration of the tin oxide layer. Also, the tin oxide layer has a significant resistance, and this resistance varies from capillary to capillary depending upon the dimensions of the layer and also varies within one capillary as the oxide layer deteriorates. The present inventors have measured this contact resistance and found it to be in the range 20-70 ohms. Other researchers have measured the resistance and claim values to several hundred ohms. Sturrock and Williams, Modified Static Mercury Drop Electrode, 54 Anal. Chem. 2629-31 (1982).
Prior art devices demonstrate other disadvantages as well. For example, one prior art device offers only three different drop sizes. Although fine control of drop surface area over a wide range provides possibilities for optimizing many different procedures on one electrode, for unattended monitoring, and for study of surface relations, this control is not generally available on prior art devices. Prior art electrodes also suffer from poor reproducibility. For example, with one commercially available model, it is difficult to reproduce mercury drop size to better than 3% from day to day. Frequent recalibration of the drop size is necessary for accurate measurements. The drop size must be known to measure the fundamental calibration constant, the diffusion coefficient. It must be maintained constant to achieve constant sensitivity.
Another problem with prior art electrodes is the fact that the housings are generally constructed of opaque material, such as stainless steel. This construction renders it impossible to visually monitor the mercury for contaminants and for proper mercury level. Moreover, it is usually not possible to invert prior art electrodes for the purpose of replacing the capillary, without first draining the mercury from the reservoir, which is a time-consuming operation. Finally, a general problem with prior art devices is that abnormal current-time behavior is obtained when the mercury is flowing out from the capillary. In practice, a waiting time of one second or longer is required for a newly formed mercury drop to stabilize before any current measurement is made.